Optimizing TDA7294 Output

[simply14prem]

Originally Posted by danielwritesbac

Fascinating! Is the source a tape head, phono cartridge, microphone or direct feed from a tuner? What music source do you have there?

I have a phone, a satellite reciever,, they are all weak without a pre amp but yes from a dvd player sound is stronger.

[simply14prem]

Do we have an inverting TDA set up ?

[danielwritesbac]

Maybe KSTR could do it. He's wanting a minimized gain TDA7294 power amp and inverting mode might help accomplish that. (most chip amplifiers can achieve lower gain when run inverting) But when I think about that, I also wonder how big and elaborate the preamp for it might be.

[Fast Eddie D]

I want to thank Daniel and everybody else that contributed to this thread.

I have mentioned my "no name" chip amp before. Well I got it out because I intend to add a preamp (tone controls etc) to it so it is a truly integrated amp. I dusted off the chip and used my new illuminated magnifying glass (my eyesight is really terrible ) and found out that I have TDA7296 chips running off a 62 volt split supply.

I used the same board that was in the "no-fi" tabletop stereo. I removed the ripple caps (2200uF per rail is all it had) and installed 330 uF caps on the board. I used a capacitor bank like Daniel's with 4x2200 uF per rail and it made a noticeable improvement. Then I swapped the bootstrap and feedback caps (it used the data sheet 22 uF caps with smaller feedback resistors - really terrible!) with 100 uF caps and it made a huge difference. (I did not even know that the chip used a bootstrap - I just swapped it out.)

It works really great, but I know there are still gains to be had. I am not going to tweak the chip amp any more; I am just going to put the preamp in and put it back in the TV room. After reading this thread for probably the 50th time, I am motivated to build a parallel-bridge amp using 4 chips per channel. It will be optimised to the max (dual monoblock etc) and I will try the shottky diode trick to isolate the power supply for the low level input circuitry. I think that 100 watts RMS into 8 ohms is a good goal to shoot for.

I am truly inspired that such performance is available from such a generic chip by optimising design and layout. That's what this "hobby" and website is all about.

[danielwritesbac]

Thanks for posting! Kudos on the refit!
I'm grateful that your post shows that a good power circuit (big enough tank for good bass + working decoupling supporting high speed transient for high resolution treble) is an excellent head start for high quality audio. For TDA729X, the datasheet example application is like a prank, and that is used in retail amplifiers; so, improving performance begins with replacing the power circuit.

I'm a bit impressed. Normally, I would have risked 220u amplifier board power caps, and that's excellent clarity at some risk of shouty (but not an impossible amount) and then re-tune the whole amplifier board to keep the clarity, dump the shout. But, for Television amplifier with TDA729X, and a speedy refit, your 330u was the perfect choice for good clarity with no risk of shout. Good job!! I'll bet it runs cooler too.

P.S.
Did you check to see if the NFB-Shunt cap was a reasonable size for clear quality bass? It is possible to make a refit bypass to fit underneath the board, but I'd need to know the size of the existing cap and its associated series resistor. If I knew that info, I could make a speedy stick-on refit circuit for it, like those press on nails, except for different purpose of high resolution bass.

[Fast Eddie D]

Originally Posted by danielwritesbac

Thanks for posting! Kudos on the refit!
I'm grateful that your post shows that a good power circuit (big enough tank for good bass + working decoupling supporting high speed transient for high resolution treble) is an excellent head start for high quality audio. For TDA729X, the datasheet example application is like a prank, and that is used in retail amplifiers; so, improving performance begins with replacing the power circuit.

I discovered the capacitor bank concept by accident. I was fabbing up a headphone amp (DoZ type topology) from old salvage parts (some are real old- more than 30 years!) and it sounded tubby and flat. I was using some really old 2200 uF salvage caps and all I had on hand was some 220 uF caps from a capacitor "kit" I bought (I always buy this stuff to have on hand) and I used 10 per amplifier (single ended power supply sorry haters)(channels isolated by simple 10 ohm resistors) and that was the money shot! I knew there was something to it and so I googled around (finally googled the term "capacitor bank") and it brought me right to some of your posts. Since I am trained in electrical engineering I was not satisfied until I knew why it worked; and it makes perfect sense.

I'm a bit impressed. Normally, I would have risked 220u amplifier board power caps, and that's excellent clarity at some risk of shouty (but not an impossible amount) and then re-tune the whole amplifier board to keep the clarity, dump the shout. But, for Television amplifier with TDA729X, and a speedy refit, your 330u was the perfect choice for good clarity with no risk of shout. Good job!! I'll bet it runs cooler too.

Both amps are on the same board so 330 uF was a logical choice. I did not try different values or optimising the rest of the circuit; I just slapped them in there because it's what I had. I also put some 0.1 uF metallized poly caps (20 in each capacitor "kit" so use em up) at the caps and at the power supply pins of each chip.

I am surprised how cool it runs. When I tried it with no mods, it sounded terrible and ran hot. Is this maybe because of gross phase errors from the really terrible original circuit?

P.S.
Did you check to see if the NFB-Shunt cap was a reasonable size for clear quality bass? It is possible to make a refit bypass to fit underneath the board, but I'd need to know the size of the existing cap and its associated series resistor. If I knew that info, I could make a speedy stick-on refit circuit for it, like those press on nails, except for different purpose of high resolution bass.

I repalced the 22 uF shunt caps with 100 uF. It gives a pole of 7.9 Hz which isn't optimal but it's going to stay that way. I will build a highly optimised circuit from scratch and see what I get. Like I said, my goal is to bridge-parallel for 100 watts RMS/ 8 ohms so I have some homework to do.

I am intrigued by your use of small resistors and/or high impedance wire to isolate the power supply board from the amp board. My training says this is completely wrong, but your explanation makes sense. I will try it.

I've been following your work for a while, Daniel. You and I think alike on many topics. They don't teach this stuff in engineering school and it is painfully obvious to me that many commercial designers don't know squat about audio or else they just don't care. Keep it up.

[danielwritesbac]

Originally Posted by Fast Eddie D

I am surprised how cool it runs. When I tried it with no mods, it sounded terrible and ran hot. Is this maybe because of gross phase errors from the really terrible original circuit?

The hot temp and bad quality was from the 2200u from the datasheet schematic, very close to the chip, disabling fast transients because of outwaiting the cap. That problem also cripples the negative feedback a bit, and makes much heat.
You fixed that problem by replacing it with the 330u. Much faster!
The amount of capacitance is unique per each amplifier design because you need only just enough to dampen the midrange for a palatable tone. So, generally, we'd first try to find out how much is insufficient (maybe 100u) and then install progressively larger caps until a suitable compromise is found. For example, I like your idea of 330u for TDA729X and Television source, since a smaller cap might be a bit shouty. But you could try 270u, 220u to see if the clarity increases while the tone remains tolerable. After that's all maxed, you can then apply the rail to rail cap to recoup the pretty tone.

Originally Posted by Fast Eddie D

I am intrigued by your use of small resistors and/or high impedance wire to isolate the power supply board from the amp board. My training says this is completely wrong, but your explanation makes sense. I will try it.

I wouldn't use small resistors for that. Actually we need stuff capable of big current. The bell wire umbilical (and filter) is because constructors would have omitted the correct part, which is schottky, like MBR1035 plus an FB73 ferrite. I tried this. It looks too weird. Nobody would use it. So, I substituted 7" bell wire, 20ga solid copper. The substitute does work and has bonus peak filtering and bonus RF filtering if you tie a knot in the umbilical, close to the edge of the amp board (like a $0 ferrite). Still, it would probably sound better with the schottky (at amp board power taps) either instead of or in addition to the bell wire. The series element at this point prevents the amplifier predrive small transients from outwaiting the power tank/reservoir capacitance. So, a good series element blocks that error. Regs or diodes would be good. But, a rather fine tuned cable might do a portion of that task (while looking conveniently orthodox).
If using a discrete parts amplifier, the decoupling technique could be more elegant if done only for the voltage stage as in Hafler DH-220.

And then there's the matter of charge/recovery speed, like this: Inexpensive little CRC supply running the big Honey Badger discrete amp.

[danielwritesbac]

Originally Posted by Fast Eddie D

I replaced the 22 uF [nfb]shunt caps with 100 uF. It gives a pole of 7.9 Hz which isn't optimal but it's going to stay that way.

There's a couple of quick options there:
You could parallel on another, identical, model 100u (to form a high end 200u), or you could decrease the input cap size until the amp plays clean.

P.S.
Thanks for the compliments. I wish I was trained as an engineer, but my calc skills are too poor and there's no nearby educational facilities for EE. Gosh I wish I knew how to use a scope. Making handmade tuners is a bit slow going without a scope. Oh the darned coils! But, as a benefit of a few wars with that, aligning already built tuners is super easy. It was really good practice for power filtering too.

[Fast Eddie D]

Originally Posted by danielwritesbac

I wouldn't use small resistors for that.

I meant the small value resistors you use in your capacitor arrays. It makes sense.

I have used resistors for decoupling input and voltage gain stages before (you always should), but never to decouple the power supply from the output stage. I can see where it would help recovery time a little by "holding back" the discarge of some of the capacitors.

My present research mirrors some of your concepts. Apparently such line of thinking is unorthodox; I am unaware of any commercial equipment that uses capacitor arrays or the tweaks we discussed. But the arrays work better than a big cap for reasons that are so obvious once you analyze it. And I got the idea of "right sizing" the on board caps from you. It all makes sense.

[danielwritesbac]

One of the guys here on this thread made the amplifier board power caps out of parallel 100u caps, and then stacked on enough until the tone leveled. I haven't tried that yet.